1. Field of the Invention
The present invention relates to a nitride-based light-emitting device and a method of manufacturing the same. More particularly, the present invention relates to a nitride-based light-emitting device including a transparent electrode made of a transparent conductive oxide having a higher work function than indium tin oxide (ITO) and a method of manufacturing the same.
2. Description of the Related Art
Transparent conductive oxide films have been currently widely applied in the fields of optoelectronics, displays, and energy industries. In particular, in the light-emitting device field, studies about transparent conductive film electrodes that serve to facilitate hole injection and light emission have been actively made by numerous domestic or foreign academic, industrial, and governmental research institutes.
Among transparent conductive oxides, indium oxide (In2O3), tin oxide (SnO2), and indium tin oxide (ITO) have been the most actively studied and developed. However, these materials have a relatively low work function, and thus, their application as transparent electrodes of gallium nitride-based light-emitting diodes may cause the following problems.
First, since the above-described transparent conductive oxides have a low work function, relative to p-type gallium nitride, a high current flow barrier is formed, which renders a smooth hole injection difficult. Therefore, realization of devices with high emission efficiency may be difficult.
Second, since the above-described transparent conductive oxides have a low transmittance for a blue light beam among light beams emitted from gallium nitride-based light-emitting diodes, it is difficult to apply them to devices emitting a short wavelength light such as blue light.
Meanwhile, to realize light-emitting devices such as light-emitting diodes or laser diodes using a nitride (e.g., gallium nitride)-based semiconductor, an ohmic contact between the semiconductor and an electrode is very important.
Such gallium nitride-based light-emitting devices are classified into top-emitting light-emitting diodes (TLEDs) and flip-chip light-emitting diodes (FCLEDs).
In currently widely used TLEDs, light is emitted from an ohmic contact layer that contacts with a p-type clad layer. Such TLEDs require a high quality ohmic contact layer due to low hole density of the p-type clad layer. In this respect, it is required that the ohmic contact layer is formed as a low resistance and high transmittance layer to compensate for low electroconductivity, thereby facilitating hole injection and light emission.
Generally, such TLEDs have a sequentially stacked structure of a nickel (Ni) layer and a gold (Au) layer on the p-type clad layer.
The Ni/Au layer has a good specific-contact resistance of 10−3 to 10−4 Ωcm3 and is used as a semi-transparent ohmic contact layer.
When the Ni/Au layer is annealed at a temperature of 500 to 600° C. and under an oxygen atmosphere, nickel oxide (NiO) which is a p-type semiconductor oxide is generated at the interface between the p-type clad layer made of gallium nitride and the Ni/Au layer used as the ohmic contact layer. Therefore, a Schottky barrier height (SBH) is decreased and supply of holes, which are the majority carriers, to a surface of the p-type clad layer is facilitated.
According to a general understanding, when the Ni/Au layer is formed on the p-type clad layer and annealed, the concentration of effective carriers on a surface of the p-type clad layer reaches 1018 or more by reactivation that increases the concentration of a magnesium dopant on the p-type clad layer by removal of an Mg-H intermetallic compound. Therefore, tunneling conduction occurs between the p-type clad layer and the ohmic contact layer, which shows ohmic conduction characteristics of low specific-contact resistance.
However, TLEDs using a semi-transparent film electrode made of Ni/Au have low emission efficiency due to presence of Au with poor light transmittance, which limits realization of next generation, large capacity and high brightness light-emitting devices.
With respect to FCLEDs that emit light from a transparent sapphire substrate using a reflective layer that serves to increase emission efficiency of heat generated during driving and light emission efficiency, there also arise many problems such as high resistance due to oxidation and poor adhesion of the reflective layer.
In view of the above-described limitations of such TLEDs and FCLEDs, there was suggested a p-type ohmic contact layer made of a transparent conductive oxide, for example ITO, which has an excellent light transmittance relative to semi-transparent Ni/Au used for a conventional p-type ohmic contact layer. The ITO ohmic contact layer can increase the output of light-emitting devices but exhibits a relatively high driving voltage. Therefore, the ITO ohmic contact layer has many limitations on its application to large area and capacity, high brightness light-emitting devices.